1. Field of the Invention
The present invention relates to an optical apparatus used in a laser printer in which image forming is performed by irradiating a laser beam which has been modulated based on image information through a scanning mirror onto a surface of a photosensitive body.
2. Description of the Prior Art
A known optical apparatus for scanning light such as a laser beam through a scanning mirror is used in a laser printer in which an image is formed by electrophotographic method in which a laser beam modulated based on an image data is scanned onto a photosensitive body surface. As shown in FIG. 8 in the interior of a main body 101 of the laser printer, a laser beam irradiated by a laser irradiating unit 123 is transmitted onto a surface of a photosensitive body drum 108 through a scanning mirror 121 and reflecting mirrors 122a to 122c. The photosensitive body drum 108 is supported in an approximately central portion of the main body 101 so as to be rotatable. The scanning mirror 121 is driven by a mirror motor 120 so as to rotate, and scans the laser beam irradiated by the laser irradiating unit 123 in a main scanning direction of the photosensitive body drum 108, that is, a direction parallel to a rotating axis of the photosensitive body drum 108. The surface of the photosensitive body drum 108 is uniformly charged with electric charge of single polarity by an electrification unit 114 prior to the transmission of the laser beam, and forms an electrostatic latent image through a photoconductive phenomenon by the laser beam modulated based on the image data. Toner in a developing vessel 116 is supplied to this electrostatic latent image from a developing roller 115 whereby the electrostatic latent image is clarified into a toner image.
On the other hand, in the interior of the main body 101 is formed a paper delivery path 113 which extends from a paper feeding tray 117 through an interval between the photosensitive body drum 108 and a transferring roller 109 and through a fixing device 110 to a paper discharge tray 118. There are arranged, within this paper delivery path 113, a paper feeding roller 103, a paper sensor 107, delivery rollers 111, and paper discharge rollers 112. Paper 102 stacked in the paper feeding tray 117 is fed into the paper delivery path 113 by the rotation of the paper feeding roller 103. The fed paper 102 is detected by the paper sensor 107. The toner image of the photosensitive body drum 108 is transferred onto the surface of the paper 102 when the laser irradiating unit 123 irradiates a laser beam based on the detection signal of the paper sensor 107 and the paper 102 is made to pass between the photosensitive body drum 108 and the transferring roller 109 synchronously with the rotation of the photosensitive body drum 108. The toner image transferred onto the surface of the paper 102 is fused and fixed on the surface of the paper 102 by heating and pressurizing operations of the fixing device 110. The paper 102 on which the toner image has been fixed is discharged to the paper discharge tray 118 by the paper discharge rollers 112. After the surface of the photosensitive body drum 108 has faced the paper 102, the surface is cleared of residual toner by means of a cleaner 126 and is thereafter reused for forming electrostatic latent images and toner images.
As shown in FIG. 9, the scanning motor 120, the scanning mirror 121, the reflecting mirrors 122a to 122c (note that the reflecting mirrors 122b and 122c are omitted from the drawing), the laser irradiating unit 123 and a start sensor 124 make up an optical unit 201 in the laser printer. The scanning mirror 121 scans a laser beam from the laser irradiating unit 123 which has been modulated based on the image data while steady-state rotation in a direction shown by arrow CW, driven by the scanning motor 120. The reflecting mirrors 122a to 122c transmit the laser beam which has been scanned by the scanning mirror 121 to the photosensitive body drum 108. The scanned laser beam is further received by the start sensor 124. Upon receipt of the laser beam, the start sensor 124 outputs a SYNC signal. This SYNC signal is used for creating reference timing for outputting image data to the laser irradiating unit 123.
FIG. 10 is a block diagram showing an arrangement of a control unit of the laser printer. The control unit of the laser printer is composed of the optical unit 201, a synthesizing circuit 202, an image data generating circuit 203 and a load device 204, which are connected with a CPU 209 having a ROM 207 and RAM 208 through a interface 205. The optical unit 201 inputs the SYNC signal output by the start sensor 124 to the image data generating circuit 203 and the CPU 209. The CPU 209 creates a LEND signal based on the SYNC signal which is then output to the synthesizing circuit 202. Based on the SYNC signal, the image data generating circuit 203 outputs the image data to the synthesizing circuit 202. The synthesizing circuit 202 outputs a video signal synthesized from the LEND signal and the image data to the laser irradiating unit 123 of the optical unit 201. The laser irradiating unit 123 irradiates a laser beam based on the Video signal.
FIG. 11 is timing charts for each signal of the control unit of the laser printer. The SYNC signal for creating the reference timing for outputting image data to the laser irradiating unit 123 may be, for instance, a low level signal output from a time at which the start sensor 124 receives the laser beam. Therefore, in order to generate the SYNC signal, a laser beam needs to be irradiated from the laser irradiating unit 123. On the other hand, in order to accurately reproduce image data by using the photosensitive body drum 108, only the laser beam modulated by the image data should be transmitted to the surface of the photosensitive body drum 108. For this purpose, the CPU 209 sets the LEND signal, which is output to the synthesizing circuit 202 during a period of time TA beginning shortly time t1 at which the SYNC signal has been input until time t2, to "H". This period of time TA is sufficient for the laser beam to cross the photosensitive body drum 108 in an axial direction.
The image data generating circuit 203, on the other hand, starts output of image data to the synthesizing circuit 202 at time t3 when a period of time TB has elapsed after time t1 at which the SYNC signal has been input. This period of time TB is a period of time in which an irradiating position of a scanning light of the laser beams moves from a position at which the light is received by the start sensor 124 to a position at which the light faces the photosensitive body drum 108. The Video signal synthesized from the LEND signal and the image data is output from the synthesizing unit 202 to the laser irradiating unit 123. It should be noted that the laser irradiating unit 123 terminates irradiation of a laser beam when the Video signal is "H" and irradiates a laser beam when the Video signal is "L".
In this manner, a Video signal of level "L" is input into the laser irradiating unit 123 for continuously irradiating a laser beam within a specified period of time when the laser beam is irradiated at a position at which it is received by the start sensor 124. On the other hand, when the laser beam is irradiated at a position at which it faces the photosensitive body drum 108, the LEND signal is set to "H" level and the Video signal corresponding to the image data is input to the laser irradiating unit 123 so that a laser beam modulated based on the image data is irradiated onto the surface of the photosensitive body drum 108. Such control of signals which are input to a light source of the laser irradiating unit or the like is similarly performed not only for laser printers but also for optical apparatuses using scanning light such as a position measuring apparatus.
In the above-described conventional optical apparatus, lapse of time is measured starting from a time at which the scanning light has been irradiated to a reference position which may be, for instance, an irradiating position at which it is received by the start sensor, and an irradiating position of the scanning light is determined from this measured lapse of time in order to supply a signal to the light source of the scanning light according to the irradiating position of the scanning light. A period of time, at which the irradiating position of the scanning light moves from the reference position to a specified position which may be, for instance, a position on the photosensitive body drum, varies in accordance with a distance between the reference position and the specified position, and a scanning speed of the scanning light which is determined by a rotating speed of the scanning mirror. The distance between the reference position and the specified position as well as the rotating speed of the scanning mirror are variable in accordance with the type and specifications of the optical apparatus, and details of the control for the light source such as driving timing for the light source of the scanning light need to be changed in accordance with the specification of the optical apparatus.
For instance, in the case where the rotating speed of the scanning motor used for rotating the scanning mirror in the laser printer is changed without making a modification to a control device, which has been used before changing the rotating speed of the scanning motor, modulation of the laser beam based on the image data starts before or after the laser beam is irradiated onto the surface of the photosensitive drum, so that not only the laser beam which has been modulated based on the image data can not be irradiated onto an accurate position of the photosensitive body drum but also the image formed on the surface of the photosensitive body drum expands or contracts in a direction parallel to the scanning direction of the laser beam with the result that the image is deformed. Further, by a change in the irradiating area of the laser beam, the laser beam is irradiated onto portions of the optical apparatus which are not supposed to be irradiated with the laser beam so that a case may occur in which parts of the laser printer are damaged or heated. Therefore, a drawback was presented in that designs of control units needed to be changed each time the specification of the optical apparatus was varied not only in the case where the type of the optical apparatus is changed but also in the case where the type of the optical apparatus is identical.
Further, in case the rotating speed of the scanning motor is changed due to a breakdown, the light source can not be accurately controlled by a predetermined timing for a control action, and due to the change in the irradiating area of the laser beam, the light beam is irradiated onto portions of the laser printer where are not supposed to be irradiated with the laser beam so that parts of the laser printer may be damaged or heated. It may further be in the laser printer that the laser beam modulated based on the image data can not be irradiated on accurate positions of the photosensitive body drum or the image formed on the surface of the photosensitive drum expands and contracts in a direction parallel to the scanning direction of the laser beam so that the image is deformed.
Another prior art concerning control of an optical apparatus is an information output apparatus disclosed in Japanese Examined Patent Publication JP-B2 3-48511 (1991). This information output apparatus corresponds to an optical unit for irradiating a laser beam onto a photosensitive drum in an electrophotographic process, and a timing for a semiconductor laser element starting output of a laser beam is controlled in order to sent an optical information from a control device to the photosensitive drum in accordance with a beam detecting signal which a beam detector outputs upon detecting a laser beam from the semiconductor laser element.
In the case where the beam detector is made to detect a laser beam, the control apparatus makes the semiconductor laser element start continuous output of a laser beam at a time which is set back by a time required for stabilizing an amount of light of the laser beam which is output by the semiconductor laser element from a time at which the beam detector should detect the laser beam in order to prevent generation of a jitter. In this information output apparatus, since a case where the control apparatus is made to control a plurality of optical apparatuses whose arrangements differ each other, has not been taken account of, no concrete arrangements for this purpose are shown therein. Therefore, signals for controlling the semiconductor laser element generated based on signals output by the beam detector always remain the same so that the design of the control device is required to be changed when the arrangement of the optical apparatus to be controlled thereby is changed.